Catalytic cracking is a petroleum refining process that is applied commercially on a very large scale. Catalytic cracking, and particularly fluid catalytic cracking (FCC), is routinely used to convert heavy hydrocarbon feedstocks to lighter products, such as gasoline and distillate range fractions. In FCC processes, a hydrocarbon feedstock is injected into the riser section of a FCC unit, where the feedstock is cracked into lighter, more valuable products upon contacting hot catalyst circulated to the riser-reactor from a catalyst regenerator.
It has been recognized that for a fluid catalytic cracking catalyst to be commercially successful, it must have commercially acceptable activity, selectivity, and stability characteristics. It must be sufficiently active to give economically attractive yields, have good selectivity towards producing products that are desired and not producing products that are undesired, and it must be sufficiently hydrothermally stable and attrition resistant to have a commercially useful life.
Excessive coke and hydrogen are undesirable in commercial catalytic cracking processes. Even small increases in the yields of these products relative to the yield of gasoline can cause significant practical problems. For example, increases in the amount of coke produced can cause undesirable increases in the heat that is generated by burning off the coke during the highly exothermic regeneration of the catalyst. Conversely, insufficient coke production can also distort the heat balance of the cracking process. In addition, in commercial refineries, expensive compressors are used to handle high volume gases, such as hydrogen. Increases in the volume of hydrogen produced, therefore, can add substantially to the capital expense of the refinery.
Improvements in cracking activity and gasoline selectivity of cracking catalysts do not necessarily go hand in hand. Thus, a cracking catalyst can have outstandingly high cracking activity, but if the activity results in a high level of conversion to coke and/or gas at the expense of gasoline the catalyst will have limited utility. Catalytic cracking in current FCC catalyst is attributable to both the zeolite and non-zeolite (e.g. matrix) components. Zeolite cracking tends to be gasoline selective, while matrix cracking tends to be less gasoline selective.
In recent years, the oil refining industry has shifted to processing a larger quantity of residual (resid) and resid-containing feeds due to changes in the price structure and availability of crude oil. Many refiners have been processing at least a portion of residual oil in their units and several now run a full residual oil cracking program. Processing resid feeds can drastically alter yields of valuable products in a negative direction relative to a light feed. Aside from operational optimizations, the catalyst has a large impact on product distribution. Several factors are important to resid catalyst design. It is highly favorable if the catalyst can minimize coke and hydrogen formation, maximize catalyst stability, and minimize deleterious contaminant selectivity due to metal contaminants in resid feedstocks.
Resid feeds typically contain contaminant metals including Ni, V, Fe, Na, Ca, and others. Resid FCC for converting heavy resid feeds with high Ni and V contaminants constitutes the fastest growing FCC segment globally. Both Ni and V catalyze unwanted dehydrogenation reactions, but Ni is an especially active dehydrogenation catalyst. Ni significantly increases H2 and coke yields. In addition to taking part in unwanted dehydrogenation reactions, V comes with other major concerns as it is highly mobile under FCC conditions and its interaction with the zeolite destroys its framework structure, which manifests itself as increased H2 and coke yields, as well as lower zeolite surface area retention. Even small amounts (e.g., 1-5 ppm) of contaminant metals in the feed deposit cumulatively on the catalyst and can result in high H2 and coke yields during FCC operation, which is a major concern for the refining industry.
Since the 1960s, most commercial fluid catalytic cracking catalysts have contained zeolites as an active component. Such catalysts have taken the form of small particles, called microspheres, containing both an active zeolite component and a non-zeolite component in the form of a high alumina, silica-alumina (aluminosilicate) matrix. The active zeolitic component is incorporated into the microspheres of the catalyst by one of two general techniques. In one technique, the zeolitic component is crystallized and then incorporated into microspheres in a separate step. In the second technique, the in situ technique, microspheres are first formed and the zeolitic component is then crystallized in the microspheres themselves to provide microspheres containing both zeolitic and non-zeolitic components. For many years a significant proportion of commercial FCC catalysts used throughout the world have been made by in situ synthesis from precursor microspheres containing kaolin that had been calcined at different severities prior to formation into microspheres by spray drying. U.S. Pat. No. 4,493,902 (“the '902 patent”), incorporated herein by reference in its entirety, discloses the manufacture of fluid cracking catalysts comprising attrition-resistant microspheres containing high Y zeolite, formed by crystallizing sodium Y zeolite in porous microspheres composed of metakaolin and spinel. The microspheres in the '902 patent contain more than about 40%, for example 50-70% by weight Y zeolite. Such catalysts can be made by crystallizing more than about 40% sodium Y zeolite in porous microspheres composed of a mixture of two different forms of chemically reactive calcined clay, namely, metakaolin (kaolin calcined to undergo a strong endothermic reaction associated with dehydroxylation) and kaolin clay calcined under conditions more severe than those used to convert kaolin to metakaolin, i.e., kaolin clay calcined to undergo the characteristic kaolin exothermic reaction, sometimes referred to as the spinel form of calcined kaolin. This characteristic kaolin exothermic reaction is sometimes referred to as kaolin calcined through its “characteristic exotherm.” The microspheres containing the two forms of calcined kaolin clay are immersed in an alkaline sodium silicate solution, which is heated, until the maximum obtainable amount of Y zeolite with faujasite structure is crystallized in the microspheres.
Fluid cracking catalysts which contain silica-alumina or alumina matrices are termed catalysts with “active matrix.” Catalysts of this type can be compared with those containing untreated clay or a large quantity of silica, which are termed “inactive matrix” catalysts. In relation to catalytic cracking, despite the apparent disadvantage in selectivity, the inclusion of aluminas or silica-alumina has been beneficial in certain circumstances. For instance when processing a hydrotreated/demetallated vacuum gas oil (hydrotreated VGO) the penalty in non-selective cracking is offset by the benefit of cracking or “upgrading” the larger feed molecules which are initially too large to fit within the rigorous confines of the zeolite pores. Once “precracked” on the alumina or silica-alumina surface, the smaller molecules may then be selectively cracked further to gasoline material over the zeolite portion of the catalyst. While one would expect that this precracking scenario might be advantageous for resid feeds, they are, unfortunately, characterized as being heavily contaminated with metals such as nickel and vanadium and, to a lesser extent, iron. When a metal such as nickel deposits on a high surface area alumina such as those found in typical FCC catalysts, it is dispersed and participates as highly active centers for the catalytic reactions which result in the formation of contaminant coke (contaminant coke refers to the coke produced discretely from reactions catalyzed by contaminant metals). This additional coke exceeds that which is acceptable by refiners. Loss of activity or selectivity of the catalyst may also occur if the metal contaminants (e.g. Ni, V) from the hydrocarbon feedstock deposit onto the catalyst. These metal contaminants are not removed by standard regeneration (burning) and contribute to high levels of hydrogen, dry gas and coke and reduce significantly the amount of gasoline that can be made.
It would be desirable to provide FCC catalyst compositions, methods of manufacture, and FCC processes that reduce coke and hydrogen yields, in particular, in feeds containing high levels of transition metals, for example, in a resid feed.